Cermet high level waste forms

ABSTRACT

A system and method for stabilizing fission products in a cermet for long term storage. The method includes forming a metal oxide precipitate, combining the metal oxide precipitate with an undissolved solid, and densifying the combined metal oxide precipitate and the undissolved solid to provide a cermet having a ceramic dispersed phase and a metallic matrix phase, wherein the metallic matrix phase includes metallic content from the undissolved solid. The undissolved solid can include fission product metals from the reprocessing of irradiated nuclear fuel. The cermet waste loading can be greater than approximately 30 percent, reducing waste volume by 50 percent or more when compared to baseline glassified articles.

This invention was made with government support under Contract No. DE-ACO5-000R22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates to cermets, and more particularly, cermets for high level waste containment.

A number of nuclear processes generate waste with high levels of radioactive decay. These by-products must generally be stored or disposed in a manner that limits their impact on the surrounding environment. Current storage methods include the fixation of waste in glass. For example, one method includes dissolving used fuel components in nitric acid, removing recyclable components, evaporating the residual waste solution, calcining the resulting solids to produce an oxide mixture, heating the oxide mixture with borosilicate glass frit to obtain a melt, and solidifying the melt in a pelletized body. An accumulation of pelletized glass can be surface stored for decades and eventually emplaced in a geologic repository for long term storage or disposal.

The above method is particularly common in the storage of fission products generated in the reprocessing of used nuclear fuel. Despite its acceptance, however, the above method suffers from a number of drawbacks. For example, glass can have less than 30% waste loading, resulting in undesirably large quantities of glass for a given quantity of fission product. In addition, glass can have poor long-term resistance to leaching, and component materials and equipment can suffer from high levels of corrosion throughout the manufacture of the pelletized glass body.

Cermets have been proposed as an alternative to glass. Cermets include various ceramic phase particles uniformly dispersed in and microencapsulated by a metal alloy matrix. Cermets can be formed by dissolving high level waste in a molten urea, precipitating and calcining all the constituents, compacting the calcine, and sintering and reduction to form a pelletized product. The metal matrix provides an improved thermal conductivity and mechanical strength compared to glass. Despite their advantages, however, conventional cermets have yet to receive widespread acceptance in the reprocessing of used nuclear fuel.

SUMMARY OF THE INVENTION

A system and method for stabilizing high level waste in an improved cermet for long term storage is provided. The method includes forming a metal oxide precipitate, combining the metal oxide precipitate with an undissolved solid, and densifying the combined metal oxide precipitate and the undissolved solid to provide a cermet having a ceramic dispersed phase within a metallic matrix phase, wherein the metallic matrix phase includes metallic content from undissolved fuel components and other waste components that are metals or easily reducible to metals. The undissolved solid can include fission product metals from the reprocessing of irradiated nuclear fuel. The cermet waste loading can be greater than approximately 30 percent, reducing waste volume by 50 percent or more when compared to baseline glassified articles.

In one embodiment, the method includes calcining the mixture to decompose the metal oxide precipitate into component oxides. The method can also include heating the mixture in a reducing atmosphere to reduce some of the metal oxides to form at least part of the metallic matrix phase. The ceramic dispersed phase can be chemically tailored to immobilize at least one preselected first radioisotope and the metallic matrix phase can be chemically tailored to immobilize at least one preselected second radioisotope.

In another embodiment, the metal oxide precipitate is formed from an aqueous solution containing waste from the reprocessed nuclear fuel, including metal oxides and metals. The metal matrix phase can include metallic content from the undissolved solids, the zircaloy alloying metal, and the used fuel assembly hardware components.

In another embodiment, a cermet is provided. The cermet includes a ceramic dispersed phase chemically tailored to immobilize at least one preselected first isotope, and a metallic matrix phase chemically tailored to immobilize at least one preselected second isotope. The metallic matrix phase includes metallic content from an undissolved solid. In one embodiment, the matrix phase is derived primarily from the undissolved solid. In another embodiment, the matrix phase is derived primarily from waste metal in a metal oxide precipitate. Total waste loading in the cermet can approach 100 percent. Without any additives, the cermet can be greater than approximately 70 ceramic by weight and less than approximately 30 percent metal by weight.

The present invention provides a cermet having improved properties over glassified waste forms and known cermets. The resulting cermet includes a ceramic phase having low leachability and a metal matrix phase providing improved strength and high thermal conductivity. The disclosed system and method facilitate cost reductions by integrating metal waste and oxide waste into one, efficient waste stream, with little or no additives to tailor the ceramic or metal phases of the resulting cermet.

These and other features and advantages of the present invention will become apparent from the following description of the invention, when viewed in accordance with the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of direct cermet formation with minimal additives in accordance with an embodiment of the present invention.

FIG. 2 is a flow diagram of a cermet preparation process in accordance with an embodiment of the present invention.

FIG. 3 is a table illustrating the phase equilibrium of a cermet manufactured in accordance with an embodiment of the present invention.

FIG. 4 is a micrograph of a low metal cermet manufactured in accordance with an embodiment of the present invention.

FIG. 5 is a micrograph of a high metal cermet manufactured in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE CURRENT EMBODIMENT

The invention as contemplated and disclosed herein includes a system and method for stabilizing fission products in a cermet for long term storage. The system and method generally include blending a calcined oxide powder with undissolved fission product metals, zircaloy alloying metals, and/or other metals prior to hydrogen reduction and hot pressing or sintering.

In one embodiment, an improved cermet includes a composite of ceramic phases encapsulated in a metal matrix. The ceramic phases immobilize at least a first radioisotope in a stable, low-leachability chemical form (e.g., non-water soluble), and the metal matrix immobilizes at least a second radioisotope. As described in greater detail below, the cermet can be formed by blending a homogeneous oxide powder with undissolved high level waste. The homogeneous oxide powder can be formed by first reacting an aqueous solution with molten urea, precipitating or evaporating the resulting constituents, and denitrating the resulting slurry. Denitration can be accomplished using any suitable method, including rotary kiln modified direct denitration and batch denitration. The homogeneous oxide powder, in a flowable form, is subsequently calcined and then blended with the undissolved fission products, zircaloy metals and/or other alloying metals. The resulting composition can be reduced to convert the denitration oxides to metal. These newly reduced metals, in addition to the previously added undissolved fission product metal waste and the added zircaloy and/or other metals, form the final alloy matrix of the cermet. Densification of the reduced composition by hot pressing or sintering forms a cermet having a greater ceramic content when compared to conventional cermets. In some embodiments, reduction is followed by densification, and in still other embodiments, both processes can be performed simultaneously. The cermet can be hot extruded as a pellet, a rod, or other configuration suitable for long term storage.

Cermet manufactured according to the present invention can be substantially more ductile and thermally conductive than glass waste forms. In addition, cermets manufactured according to the present invention can have high chemical stability, stability to radiation, and stability to temperature, while at the same to exhibiting a resistance to leaching and corrosion. When used in conjunction with nuclear processes, the cermets can isolate radioactive isotopes from oxide waste and metal waste. The present method is suitably adapted to store high level waste not otherwise burned or recycled. As shown in FIG. 1, the direct cermet formation can include component zircaloy cladding, alloying metals and used fuel assembly hardware, and can also include fission products associated with used nuclear fuel such as metals, rare earth elements, cesium, rubidium, strontium, barium and zirconium, for example. As also shown in FIG. 1, the resulting pelletized cermet waste forms can be received within one or more storage cylinders for long term storage in a repository. The remaining zirconium components and spent uranium can be separately processed or recycled, and limited concentrates of actinides can be burned according to known methods. The residual alloying metals after zirconium recovery and the neutron activated structural hardware still results in a waste form composed of greater than 90% waste if no virgin material is added to tailor the phases.

One method for manufacturing the cermet of the present invention is shown in connection with the flow diagram of FIG. 2. According to this method, an aqueous solution including metal oxides in nitric acid is provided. The aqueous solution can also include heavy metal nitrates, sodium nitrates, and actinide concentrates. At step 20, excess water is driven from the aqueous raffinate solution in an evaporation process.

For example, the evaporation process can include heating the raffinate to approximately 140° C. for one hour to reduce the water content to between 40% and 80% by weight. Optionally, decomposable additives are added to the dehydrated raffinate in feed preparation at step 22 to form a slurry feed. The additives can include aluminum, silicon and/or titanium in soluble forms, though other additives can also be utilized. The additives can be added to the slurry feed to tailor the phase of the resulting denitration oxide, to enhance its corrosion resistance, or to enhance other properties of the cermet.

The slurry feed can include ceramic phase particles and metal phase particles. For example, the ceramic phase particles can include aluminum oxide, zirconium oxide, hafnium oxide, vanadium oxide, aluminum nitride, zirconium nitride, hafnium nitride, vanadium nitride, aluminum boride, zirconium boride, hafnium boride, vanadium boride, aluminum silicate, zirconium silicate, hafnium silicate, vanadium silicate, beryllium oxide, beryllium nitride, beryllium borate, beryllium silicate and combinations thereof. In addition, the metal phase particles can include titanium, aluminum, magnesium, nickel, copper, iron, cobalt, molybdenum, niobium, zirconium, and combinations thereof.

In order to precipitate the metal oxides in the slurry feed, the slurry feed is denitrated at step 24. Denitration can include heating the slurry in modified direct denitration rotary kiln as set forth in U.S. Pat. 4,409,157 to Haas et al, the disclosure of which is incorporated by reference in its entirety, though other denitration processes can also be utilized. Optionally, denitration includes raising the temperature of the slurry to about 180° C. for 40 to 50 minutes. Denitration can yield gaseous products and solid precipitates of hydrated metal oxides. The hydrated metal oxides can precipitate homogeneously, though this is not strictly required. Eventually a flowable powder precipitate is formed, where the metal oxides, optional fission products and optional decomposable additives are distributed somewhat evenly throughout the precipitate.

At step 26, the raffinate denitrated oxide precipitate is calcined to decompose the solid precipitates into their component oxides. Calcining can occur in air at 800° C. At step 28, the calcined product is combined or blended with the undissolved solids, optionally including ceramic and/or metal formers. In addition, the calcined product can be combined or blended with zircaloy alloying metals, other alloying metals, and metals from hardware disassembly and recycle, for example. At step 30, compacts of the resulting blend are confined to a furnace capable of retaining an atmosphere of a reducing gas (e.g., hydrogen or carbon monoxide) in a controlled concentration. For example, the dried powder blend can be reduced in a two step process at 800° C. to 850° C. for one hour in a hydrogen atmosphere to convert the reducible metal oxides to metal from which the final alloy matrix is at least partially derived. The undissolved solids, zircaloy alloying metals, and assembly hardware may form part or most of the final alloy matrix. At step 30, the reduced mixture is consolidated by hot pressing or sintering. For example, the reduced mixture can be hot pressed at 1100° C. and 4000 PSI for one hour to form a pelletized cermet waste immobilization article containing fission products surrounded by a metal matrix.

A phase equilibrium table is shown in FIG. 3 for the cermet composition after the aforementioned calcining, reduction, and hot pressing operations without the inclusion of additional waste metals. Surrogate high level waste cermet pellets manufactured according to the above method (without the addition of the metal waste stream) were determined to include less than approximately 30% metal and greater than approximately 70% ceramic by weight. Assuming no change in the ceramic phase, an increase in metal phase to 70% by weight can achieve an improved raffinate waste loading of greater than approximately 40% and a total waste loading (including the metal waste) of greater than 90%. Such cermets provide pellet strength greater than the strength of glass waste pellets, thermal conductivity greater than the thermal conductivity of glass waste pellets, an enhanced leach resistance over glass waste pellets.

Scanning electron microscope (SEM) micrographs of low and high metal cermet samples are shown in FIGS. 4 and 5, respectively. The cermet pellet included a ceramic dispersed phase chemically tailored to immobilize at least one preselected first radioisotope, for example Cs, Ba and Sr. The metal matrix phase is tailored to immobilize at least a second preselected radioisotope, for example Mo, Tc, Ru, Rh and Pd. Waste loading levels of the surrogate high level waste cermet pellets were determined by the waste composition, and also using the metal contents of the waste. A waste loading of at least 30% was achieved, contributing to a volume reduction of at least 50% when compared to glass waste pellets. The infusion of additives for compositional adjustments can reduce the amount of radioactive waste loading that can be accomplished. In the present embodiment, however, no additives were used to tailor the ceramic or metal phases, thereby enhancing the waste loading of the cermet pellet.

The above embodiments provide a system and method for the manufacture of cermets suitable for use in many industrial and nuclear applications. The above embodiments facilitate cost reductions by simplifying and integrating processing steps. Also, the above embodiments integrate metal waste and oxide waste into one, efficient waste stream. Using the aforementioned process, cermets can be manufactured in a manner considerably more efficient and cost effective than previously known.

The above description is that of current embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. Any reference to elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” is not to be construed as limiting the element to the singular. 

1. A method for fabricating a cermet comprising: forming a metal oxide precipitate; combining the precipitate with an undissolved solid to form a mixture; and densifying the mixture to provide a cermet having a ceramic dispersed phase and a metallic matrix phase, wherein the metallic matrix phase includes metallic content from the undissolved solid.
 2. The method of claim 1 wherein the cermet includes a waste loading greater than approximately 30 percent.
 3. The method of claim 1 wherein the precipitate is calcined prior to combining and densifying.
 4. The method of claim 1 further including heating the mixture in a reducing atmosphere to a temperature sufficient to form at least part of the metallic matrix phase.
 5. The method of claim 1 wherein the undissolved solid includes fission product metals from the reprocessing of nuclear fuel.
 6. The method of claim 1 wherein the metal oxide precipitate is formed from an aqueous solution containing waste from the reprocessing of nuclear fuel.
 7. The method of claim 1 wherein the ceramic dispersed phase is chemically tailored to immobilize at least one preselected first radioisotope and the metallic matrix phase is chemically tailored to immobilize at least one preselected second radioisotope.
 8. A cermet for isolating high level waste comprising: a ceramic dispersed phase chemically tailored to immobilize at least one preselected first isotope; and a metallic matrix phase chemically tailored to immobilize at least one preselected second isotope, wherein the metallic matrix phase includes metallic content from an undissolved solid.
 9. The cermet of claim 8 wherein the cermet waste loading is greater than approximately 30 percent.
 10. The cermet of claim 8 wherein the cermet is greater than approximately 70 percent ceramic by weight and less than approximately 30 percent metal by weight.
 11. The cermet of claim 8 wherein the metallic matrix phase is derived primarily from the undissolved solid.
 12. The cermet of claim 8 wherein the undissolved solid includes fission product metals from the reprocessing of irradiated nuclear fuel.
 13. The cermet of claim 8 wherein the undissolved solid includes zircaloy alloying metal.
 14. A method of making a cermet comprising: providing a slurry comprising ceramic phase particles and metal phase particles; denitrating the slurry to form a metal oxide precipitate; combining the metal oxide precipitate with an undissolved fission product; and densifying the combined metal oxide precipitate and undissolved fission product to provide a cermet having a ceramic dispersed phase and a metallic matrix phase.
 15. The method of claim 14 wherein the slurry is formed from an aqueous solution containing metal oxides from the reprocessing of irradiated nuclear fuel.
 16. The method of claim 14 further including calcining the metal oxide precipitate to decompose the metal oxide precipitate into component oxides.
 17. The method of claim 16 further including heating the combination in a reducing atmosphere to a temperature sufficient to form the metallic matrix phase.
 18. The method of claim 14 wherein the undissolved fission products include an alloying metal.
 19. The method of claim 18 wherein the alloying metal includes zircaloy.
 20. The method of claim 14 wherein the ceramic dispersed phase is chemically tailored to immobilize at least one preselected first radioisotope and the metallic matrix phase is chemically tailored to immobilize at least one preselected second radioisotope. 